The present invention relates to the field of optical communications and, more specifically, to the modulation of optical pulse streams. In particular, the modulation of the pulse streams is obtained by applying controlled delays to the optical pulses in the pulse stream.
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to transmit an analog optical signal is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, an optical pulse stream samples an analog signal. A temporal shift in the pulse position of each optical pulse represents a sample of the transmitted waveform. Thus, the temporal position of each pulse is shifted from its unmodulated position in proportion to the amplitude of the analog signal. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude-modulated signal is shown below:
SNRPPMSNRAM(tp/xcfx84)2xe2x80x83xe2x80x83Eq.1
where tp is the temporal spacing between unmodulated pulses and xcfx84 is the pulse duration of each pulse, respectively.
Therefore, the optical pulses used for PPM should be of short duration since SNR performance improves as the pulse widths within the modulated pulse stream decrease. Pulse widths as short as 0.3 picoseconds may be desirable for a PPM optical communication system. However, it is also well known in the art that PPM performance will suffer if the shapes of the optical pulses vary or the amplitudes of the pulses vary on a pulse-to-pulse basis. Mode locking of a pulsed laser is a mature technique for producing equally spaced ultra-short identical pulses. It would be beneficial to use a mode locked laser in a PPM communication system if the equally spaced pulses produced by the system could be modulated without distortion.
Furthermore, the PPM system should be capable of supporting the modulation and transmission of analog signals with large bandwidths. Typically, a bandwidth of xcex94f=1-10 GHz and higher is of interest for inter-satellite communications. Since pulse repetition frequencies (PRF) of 1/tp greater than 2xcex94f are required for sampling a signal with a bandwidth of xcex94f, trains of picosecond pulses with a PRF over one gigahertz should be used for realizing the advantages of PPM. For example, an optical inter-satellite link designed to transmit waveforms with a bandwidth xcex94f=20 GHz requires a sampling rate with a PRF=1/tpxe2x89xa72xcex94f=40 GHz. At a sampling rate of 40 GHz and optical pulses with 1 picosecond duration, a 30 dB gain is realized over an AM system with equal optical power.
Implementations of PPM for optical communications require a mechanism for modulating the delays between extremely short optical pulses within a pulse stream without modulating the shapes or pulse-to-pulse amplitudes of the pulses. Direct modulation of a semiconductor laser will appropriately modulate the delay between the optical pulses generated by the laser. However, a directly modulated semiconductor laser generates relatively long pulses that result in limited SNR performance. Pulse compression can be used on the longer pulses produced by the directly modulated semiconductor laser, but devices to provide such compression are complex and cumbersome. Direct modulation of a semiconductor laser may also introduce amplitude modulation or pulse reshaping of the individual time-shifted pulses, further limiting performance.
Pulse position modulation of extremely short optical pulses is also achieved by applying a pulse-to-pulse delay external to the source of the equally spaced optical pulses. That is, a modulator is used that can receive a stream of optical pulses, change the pulse-to-pulse delay at the rate required for properly sampling the transmitted analog signal, and further transmit the delayed pulses. It is known in the art that one example of a pulse position modulator for optical pulses consists of an optical delay line, such as a parallel slab of transparent electro-optically active material. The refractive index of the electro-optically active material can be controllably varied by an applied voltage, so that each pulse is controllably delayed upon traversing the electro-optically active material in accordance with the instantaneous voltage. However, such a modulator requires an undesirably large amount of electrical power, due to the relatively large voltages required to modulate the refractive index of the material and thus modulate the delay encountered by a pulse traversing the material.
Another example of a pulse position optical modulator relying upon the use of electro-optically active material is disclosed in U.S. Pat. No. 3,961,841, issued Jun. 8, 1976 to Giordmaine. Giordmaine discloses a device for optical pulse position modulation comprising a diffraction grating in combination with an electro-optic prism and a lens. The diffraction grating splits an incident light pulse into its frequency components and the lens directs the components into the prism. The refractive index change provided by the prism causes a phase shift in the frequency components and thus a time shift in the optical pulse once it is reconstructed by the diffraction grating. The device disclosed by Giordmaine provides the capability of modulating light pulses as short as one picosecond. However, the maximum controllable delay is limited to a few picoseconds for a 3 picosecond pulse and further decreases for shorter pulses. Also, the multiplicity of optical elements such as the diffraction grating, lens, and prism increase the complexity and manufacturing cost of the device.
A device for delaying optical pulses is disclosed in U. S. Pat. No. 5,751,466, issued May 12, 1998 to Dowling et al. and is shown in FIG. 1. Dowling discloses a photonic bandgap structure comprising a plurality of cells 18A-18N of width d in which the refractive index varies. The refractive index variation may be such that each cell comprises two layers of materials with two different indices of refraction n1 and n2. If the widths of the two layers within each cell are xcex/4n1 and xcex/4n2 where xcex is the free space wavelength of the optical pulse to be delayed, a distributed Bragg reflector structure is created. According to Dowling, the thickness and/or number of layers in the photonic bandgap structure and/or their indices of refraction are selected to produce a structure with a transmission resonance center frequency and bandwidth corresponding to the frequency and bandwidth of the optical pulse to be delayed. By matching the transmission resonance to the optical pulse, a controllable delay is imparted to the optical pulse without significantly altering the optical signal.
The device disclosed by Dowling requires that the thickness of each layer in the device be approximately one-half the wavelength of the incident optical pulse to form the photonic bandgap structure. The delay imparted on an optical signal by transmission through the structure will depend upon the number of layers and the indices of refraction within the layers. The structure can be thought of as essentially increasing the length of the waveguide in which it is contained, thus providing the desired delay. For example, Dowling discloses a simulation of a photonic bandgap structure that is 7 xcexcm thick that provides a delay equivalent to an optical signal traveling through a 110 xcexcm structure, or a delay of about 0.4 picoseconds. Since the amount of delay from a single structure is relatively small, Dowling discloses that the structures can be successively coupled in a single device to provide additional delay. Of course, this increases the overall size of the device.
Dowling also discloses that changing the indices of refraction within the layers of the structure can vary the delay provided by a photonic bandgap structure. One way to accomplish this is to fabricate at least one of the layers from electro-optically active material. An applied voltage will then change the index of refraction in the layer to which the voltage is applied. FIG. 1 shows a voltage means 15 that applies a voltage to one or more of the layers within the device disclosed by Dowling. Varying the voltage would vary the delay, thus providing the controllable delay required for pulse position modulation. However, since the overall delay provided by the photonic bandgap structure is relatively small, it would follow that the change of delay provided by electro-optically changing the indices of refraction would only be some fraction, typically 0.1% or less, of that relatively small delay. Again, this limitation could be overcome by coupling successive structures, with a corresponding increase in the overall size of the structure.
Optical transmission media can be configured to reflect optical beams at specified wavelengths. One such configuration is accomplished through the use of a xe2x80x9cBragg gratingxe2x80x9d or xe2x80x9cdistributed Bragg reflector.xe2x80x9d A distributed Bragg reflector is provided by periodic variations in the refractive index of the optical media. The pattern of variations behaves as a spectrally selective reflector for electromagnetic radiation. The reflection of a distributed Bragg reflector reaches its maximum at the wavelength xcex satisfying the Bragg condition:
xcex2(xcex)=xcfx80/xcex9xe2x80x83xe2x80x83Eq. 2
where xcex2(xcex) is the wave number at the given wavelength and xcex9 is the period of modulation of the distributed Bragg reflector.
A xe2x80x9cchirpedxe2x80x9d distributed Bragg reflector is provided in optical media by quasiperiodic variations in the refractive index with the optical media. In a chirped distributed Bragg reflector (C-DBR), the period of the refractive index variation is not a constant, but instead changes in a predetermined fashion along the propagation axis of the C-DBR. The propagation axis of a C-DBR is the direction in which incident light travels in the optical media. A specific quasiperiodic variation in the refractive index is one in which the period of the refractive index variation increases or decreases as an approximately linear function of position along the propagation axis, resulting in a linearly chirped distributed Bragg reflector. FIG. 3 shows a linearly chirped variation of the refractive index n as a function of position z along the propagation axis.
An electro-optic delay generator based on the use of a chirped distributed Bragg reflector is disclosed in the pending U.S. patent application Ser. No. 09/545,632, xe2x80x9cMethod and Apparatus for Electro-optic Delay Generation of Optical Signals,xe2x80x9d filed Apr. 7, 2000 and incorporated herein by reference. A delay generator 200 based on a C-DBR structure in an electro-optically active waveguide 120 is shown in FIG. 2. The refractive index of the electro-optically active layer 107 within the waveguide 120 is controlled by an electric field generated by electrodes 105, 106 disposed on both sides of the layer 107. In this delay generator 200, an optical pulse 31a is directed into the waveguide 120, is then reflected by the C-DBR structure within the waveguide 120, and is then directed out of the waveguide 120 as a delayed optical pulse 31b. The optical pulse is, therefore, delayed by the round trip travel time by the propagation of the pulse within the waveguide 120 to the C-DBR reflection point (as indicated by reference number 108 in FIG. 2) and the propagation of the pulse from the C-DBR reflection point 108 back out of the waveguide 120. Shifting the C-DBR reflection point 108 by using the electro-optic effect to change the refractive index within the waveguide 120 controls the delay of the optical pulse. The electro-optic effect arises due to the electric field generated between the two electrodes 105, 106 and connected to a voltage generator 109.
The delay generator 200 shown in FIG. 2 enables large (up to 10 picoseconds) temporal shifts in optical pulses. The C-DBR structure may be easily manufactured by doping lithium niobate with titanium. However the bandwidth of the delay generator 200 is limited, though, by the use of a constant modulation field applied by the bias voltage Vbias during round-trip propagation of the optical pulse. That is, the bias voltage to apply a specific delay to a specific optical pulse can not be changed until that optical pulse has entered and exited the waveguide. This limits the bandwidth of a delay generator as shown in FIG. 2 with a length of 1 cm to a few Gigahertz.
The bandwidth of the delay generator 200 shown in FIG. 2 and described above may be increased by turning the electrodes into a matched RF transmission line, where the modulating field propagates at the same speed as the optical pulse in the electro-optically active layer. Thus, the forward propagating optical pulse always sees a constant modulating field that applies the correct delay to that pulse. Since the modulating field tracks the forward propagating pulse, there is no need to slow the change in the field until the pulse has exited the waveguide. The reflected pulse, however, experiences the incorrect electric field for the desired delay, which results in unwanted optical pulse shift and pulse broadening.
Numerical modeling was used to determine the magnitude of this effect. The model was generated based on a 1 picosecond optical pulse propagating in a 1 cm long device made of lithium niobate. The applied bias voltage corresponded to a desired temporal shift of 6 picoseconds. It was found that the dominant effect is an unwanted change in the desired phase shift, whereas the pulse broadening is relatively insignificant. The magnitude of the unwanted temporal shift versus the modulating frequency is shown in FIG. 4. Such unwanted temporal shifts distort the transmitted waveform and, thereby, reduce the fidelity of any pulse position modulation system incorporating such a delay generator using matched RF transmission lines as electrodes.
Therefore, there exists a need in the art for an delay generator for optical signals that provides for handling a wider bandwidth of optical signals without causing unwanted pulse shift or pulse broadening.
Accordingly, it is an object of the present invention to provide apparatus and methods for providing controllable delays to optical signals.
It is another object of the present invention to provide delay generation that may vary the controllable delays applied to individual optical signals over a wide bandwidth. It is a further object of the present invention to provide for wide bandwidth delay generation without causing unwanted temporal optical signal shifts or broadening of the optical signals.
These and other objects are provided by coupling optical signals into an optical structure comprising a first electro-optically active waveguide and a second waveguide disposed adjacent to the first waveguide. A chirped distributed Bragg reflector structure is formed within the first waveguide. The chirped distributed Bragg reflector will reflect optical signals into the adjacent waveguide at a reflection point based upon the wavelengths of the optical signals and the chirp of the reflector structure. The resonant conditions in the C-DBR are such that reflection cannot occur back into the same waveguide. Rather, the resonance forms the reflected beam in the adjacent guide. An electric field is applied across the first waveguide. Changes in the electric field intensity cause the index of refraction of the electro-optically active material to change, which shifts the reflection point for optical signals. A shift in the reflection point results in a shift in the amount of delay applied to an optical signal. Thus, the second waveguide produces delayed versions of the optical signals coupled into the first waveguide.
Preferably, the second waveguide is disposed such that no electric field is applied across it or is fabricated from non-electro-optically active material, so that changes in the electric field intensity only affect the index of refraction within the first waveguide. It is also preferred that the electric field is applied to the first waveguide by a pair of electrodes disposed such that the electro-optically active material is located between them and the electrodes project in the same direction as the direction of optical signal propagation within the first waveguide. Preferably, the electrodes are disposed so as to form a matched RF transmission line, in which the group velocity for RF signals applied to the transmission line is approximately equal to the group velocity for optical signals within the first waveguide.
An apparatus according to an embodiment of the present invention for delaying optical signals is provided by an optical delay generator comprising: a waveguide layer comprising: a first optical waveguide comprising electro-optically active material, said first waveguide having a longitudinal axis, wherein optical signals propagate within said waveguide in a direction substantially parallel to said longitudinal axis; a chirped distributed Bragg reflector structure formed within said first waveguide, said chirped distributed Bragg reflector structure having a direction of propagation substantially parallel to said longitudinal axis and reflecting optical signals in a direction substantially opposite to said direction of propagation; a second optical waveguide disposed adjacent to and substantially parallel to said first optical waveguide, said second optical waveguide resonantly coupled to said first optical waveguide to receive optical signals reflected by said chirped distributed Bragg reflector structure; an insulating layer disposed above said waveguide layer; a ground electrode disposed on said insulating layer and positioned above said second optical waveguide; and a signal electrode disposed on said insulating layer and positioned such that the first waveguide is located between and below said ground electrode and said signal electrode, said signal electrode being electrically isolated from said ground electrode.
A method for delaying optical signals according to an embodiment of the present invention is provided by the steps of: coupling optical signals into a first waveguide comprising electro-optically active material, said first waveguide having a chirped distributed Bragg reflector structure formed within said electro-optically active material, said chirped distributed Bragg reflector structure reflecting said optical signals at a reflection point within said structure to provide reflected optical signals; applying an electric field across said first waveguide to change an index of refraction of said electro-optically active material so as to change the position of said reflection point; coupling said reflected optical signals into a second waveguide, said second waveguide resonantly coupled to said first waveguide; and, directing said reflected optical signals out of said second waveguide to provide delayed optical signals.
Embodiments of the present invention provide pulse position modulated optical signals by directing a stream of equally spaced optical pulses into an optical delay generator according to the present invention. An electrical control signal applied to the delay generator causes each optical pulse to obtain a delay proportional to the control signal. The delayed optical pulses are then directed out of the optical delay generator. The delayed optical pulses, with reduced undesired pulse shift or pulse broadening, represent an optical pulse positioned modulated version of the control signal